Variation in Articular Cartilage Thickness Among Extant Salamanders and Implications for Limb Function in Stem Tetrapods

The size and shape of articular cartilage in the limbs of extant vertebrates are highly variable, yet they are critical for understanding joint and limb function in an evolutionary context. For example, inferences about unpreserved articular cartilage in early tetrapods have implications for how limb length, joint range of motion, and muscle leverage changed over the tetrapod water-land transition. Extant salamanders, which are often used as functional models for early limbed vertebrates, have much thicker articular cartilage than most vertebrate groups, but the exact proportion of cartilage and how it varies across salamander species is unknown. I aimed to quantify this variation in a sample of 13 salamanders representing a broad range of sizes, modes of life, and genera. Using contrast-enhanced micro-CT, cartilage dimensions and bone length were measured non-destructively in the humerus, radius, ulna, femur, tibia, and fibula of each specimen. Cartilage correction factors were calculated as the combined thickness of the proximal and distal cartilages divided by the length of the bony shaft. Articular cartilage added about 30% to the length of the long bones on average. Cartilage was significantly thicker in aquatic salamanders (42 ± 14% in the humerus and 35 ± 8 in the femur) than in terrestrial salamanders (21 ± 7% in both humerus and femur). There was no consistent relationship between relative cartilage thickness and body size or phylogenetic relatedness. In addition to contributing to limb length, cartilage caps increased the width and breadth of the epiphyses by amounts that varied widely across taxa. To predict the effect of salamander-like cartilage correction factors on muscle leverage, a simplified model of the hindlimb of the Devonian stem tetrapod Acanthostega was built. In this model, the lever arms of muscles that cross the hip at an oblique angle to the femur was increased by up to six centimeters. Future reconstructions of osteological range of motion and muscle leverage in stem tetrapods and stem amphibians can be made more rigorous by explicitly considering the possible effects of unpreserved cartilage and justifying assumptions based on available data from extant taxa, including aquatic and terrestrial salamanders.

Non-invasive methods to measure inter-renal function in aquatic salamanders—correlating fecal corticosterone to the environmental and physiologic conditions of captive Necturus

We measured faecal corticosterone metabolites (fCMs) in aquatic salamander Necturus to understand how inter-renal function changes throughout seasonal environmental changes and associated reproductive activity. fCMs elevated with lower water temperatures and breeding activity. To overcome some challenges associated with faecal hormone collection/analysis, waterborne corticosterone (wCM) was validated through ACTH challenge.

Scavenging Behavior of the Aquatic Eastern Hellbender Salamander (Cryptobranchus Alleganiensis) in North Carolina

Vertebrates may scavenge, or utilize carrion resources more than has been widely assumed (Devault et al. 2003). While documentation of scavengers in terrestrial and marine systems has received increased attention (Beasley et al. 2015; Davenport et al. 2016), little is known regarding scavengers in freshwater stream systems. Dominant fully aquatic salamanders are important foragers in these systems and may rival the biomass of predatory fish species (Davic and Welsh 2004). One fully aquatic salamander in Appalachian streams, the eastern hellbender (Cryptobranchus alleganiensis), occurs in NC streams which receive potentially large seasonal influxes of hatchery reared trout (NCWRC 2017). Moreover, few reports exist in the published literature on the food preferences of eastern hellbender salamanders. While hellbender diet is comprised of 90% crayfish based on stomach content analysis (Peterson et al. 1989), a small portion of their diet is thought to include fish or fish bait (Nickerson and Mays 1973). Barring a few anecdotal reports from fisherman, little is known regarding scavenging by hellbenders in NC.

Trap characteristics and species morphology explain size-biased sampling of two salamander species

Demographic studies often depend on sampling techniques providing representative samples from populations. However, the sequence of events leading up to a successful capture or detection is susceptible to biases introduced through individual-level behaviour or physiology. Passive sampling techniques may be especially prone to sampling bias caused by size-related phenomena (e.g., physical limitations on trap entrance). We tested for size-biased sampling among five types of passive traps using a 9-year data set for two species of aquatic salamanders that have a 20 and 61 fold change in length over their ontogeny (Amphiuma means, Siren lacertina). Size-biased trapping was evident for both species, with body size distributions (body length mean and SD) of captured individuals differing among sampling techniques. Because our two species differed in girth at similar lengths, we were able to show that size biases (in length) were most likely caused by girth limitations on trap entry rates, and potentially by differences in retention rates. Accounting for the biases of sampling techniques may be critical when assessing current population status and demographic change.